Forum rules
Discussion of diving methods and equipment available prior to the development of BCDs beyond the horse collar. This forum is dedicated to the pre-1970 diving.
User avatar
ScubaLawyer
Master Diver
Posts: 1650
Joined: Thu Apr 08, 2010 11:25 am
First Name: Mark
Location: Laguna Beach, CA

Spencer-Tucker Constant Pressure Mistral Air Delivery System (STCPMADS)

Thu May 09, 2024 1:41 am

Spencer-Tucker Constant Pressure Mistral Air Delivery System (STCPMADS)

So I’m swimming into the beach from a dive the other day. My regulator is my trusty 1958 US Diver’s single-stage Mistral. My tank pressure is down to 500psi and the reg is breathing fantastically because, being an upstream design, the lower the tank pressure, the less force needed for the main diaphragm to push down on the oversize levers, push on the pin, and provide a supply of air. I only wished I could have that same ease of breathing at the beginning of the dive on a full tank as well as at the end on a tank with 500psi left.

As I’m taking my fins off and walking toward the beach a lightbulb goes off in my brain. I am sure I’m not the first to think of this, but I hadn’t heard of it before. Anyway, why not have a first stage regulator that supplies a constant IP of 500 psi leading to the single stage Mistral? That way I get the Mistral’s famous ease of breathing at lower tank pressure throughout the dive using its giant levers. Yes, I know that this actually makes it a two-stage regulator but that’s not the point. As many of us know, even with a balanced DH regulator the 2nd stage lever is smaller than in a single stage reg and doesn’t have the advantage of a fulcrum point to make things easier as in a single stage reg.

Next I called Luis and said ok, what 1st stage single hose reg will get me an IP of 500 psi? Luis said none that he was aware of. He was correct. So next I call BT and describe my plan to him. We both actually start doing some experimenting to see how high a stable IP we can get with a scuba 1st stage. The answer is not very high. I managed to get an old Poseidon 1st stage up to 200 psi and Bill slightly less. Most were around 150-160psi max even with shims in a piston reg. However, the Mistral does not fare very well at such very low standard IP’s. The Mistral needs a minimum of 500 psi input to actually function well. Luis is always right! Back to the drawing board.

Contemporaneously Bill and I came up with the idea of using an air regulator that is used on compressors. I was thinking that many of us have bench test regs with adjustable IP. August Industries sells them. The input can be has high as 6000 psi and the IP set and locked to any pressure including 500 psi. Bill used what I believe is an unbalanced reg and I plan on using a balanced one. It really makes no difference because even on the unbalanced reg the IP swing is only about a 100 psi from full tank to a low on air tank. Easy for a Mistral to handle.

I put together a parts list as follows.

Image

From my original design we dumped the Shrader valve as unnecessary and potentially a source of air constriction. My thought was to put a hose (that would tolerate 500 psi) between the reg and the yoke to the Mistral. Everything uses 1/4" npt threads.

Image

Here is what Bill came up with initially using a hose. Testing showed .4 inches on the Magnahelic. Awesome!

Image

That idea got dumped in favor of an all-in-one-unit that Bill built.

Image
Image
Image
Image
Image
Image

The Mistral sits slightly high and to the left but fits comfortably against the back. Bill took it 9 feet down in the pool and said it worked great! I plan on having mine on a set of twin 38's so I am working on plumbing the yoke downward and directly below the center tank valve.

Anyway, a fun project. I’ll let Bill chime in with his more knowledgeable test-bench jargon. :)

M
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

User avatar
SurfLung
Master Diver
Posts: 1766
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Spencer-Tucker Constant Pressure Mistral Air Delivery System (STCPMADS)

Mon May 13, 2024 11:32 am

VERY cool and fun idea. Good going Mark and Bill... I think I want one of those just for fun! :D

Now we need one to do the opposite for Voit 50 Fathom regs... Deliver high pressure when the tank is low... Ha!
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

User avatar
antique diver
Master Diver
Posts: 2211
Joined: Sun Dec 10, 2006 12:50 pm
First Name: Bill
Location: North-Central Texas

Re: Spencer-Tucker Constant Pressure Mistral Air Delivery System (STCPMADS)

Mon May 13, 2024 12:19 pm

SurfLung wrote:
Mon May 13, 2024 11:32 am
VERY cool and fun idea. Good going Mark and Bill... I think I want one of those just for fun! :D

Now we need one to do the opposite for Voit 50 Fathom regs... Deliver high pressure when the tank is low... Ha!
That Voit thing hurts my brain. I don't think I can do that. :roll:
The older I get the better I was.

User avatar
rhwestfall
Master Diver
Posts: 632
Joined: Thu Jan 16, 2014 2:34 pm
First Name: Robert
Location: "La Grande Ile"

Re: Spencer-Tucker Constant Pressure Mistral Air Delivery System (STCPMADS)

Mon May 13, 2024 2:20 pm

:lol:
Bob

No Longer Awaiting my Kraken.....

Return to “Classic Vintage Diving”

Who is online

Users browsing this forum: No registered users and 31 guests